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Friction and wear properties of rhenium
WEAR
FRICTION
313
AND WEAR
PROPERTIES
OF RHENIUM
E. KABINOWICZ Department (U.S.A.)
of Mechanical
Engineering, Massachusetts
Institute
of Technology,
Cambridge,
Mass.
(Received November zg, 1966)
SUMMARY
Rhenium is a hard metal with a hexagonal crystal structure and high ratio of c to a lattice dimensions. Friction and wear tests show that, in agreement with theoretical predictions, rhenium gives low friction and very low wear, sliding either against itself or against other metals. Rhenium can be electroplated, and lubricants are effective on this metal. INTRODUCTION
It has been known since the classical work of ERNST AND MERCHANTI that metals with a hexagonal structure have unusual frictional properties, in that low values of friction coefficient values of about 0.5 are observed, even when such a metal slides on itself, unlubricated, in air. In contrast, other metals generally give friction coefficient values of about 1.0 under similar conditions. Later work by SIKORSKI~showed that hexagonal metals also give low values of adhesion coefficient, thus confirming the unusual and favorable position of hexagonal metals in sliding combinations. However, other studies suggested that the low friction-coefficient values for hexagonal metals like zinc and titanium were accompanied by quite high wear rates, thus throwing doubt on the value of these metals3. Recently, BUCKLEY AND JOHNSON~have shown that for low friction and low wear, a metal should have, not only a hexagonal crystal structure, but a high ratio of c to a dimensions in its crystal lattice. Thus cobalt, with a c/a ratio of 1.625, gives low friction, while titanium, with a lower c/a ratio of 1.58, has distinctly poorer friction properties. A little study of the structural properties of elemental metals shows up the promising position of the metal rhenium *. It is one of the strongest of all metals, as is shown by examining the values of the two indicative structure-insensitive properties of melting temperature and Young’s modulus 5. Rhenium’s melting temperature is second only to tungsten’s, while in terms of a high value of Young’s modulus rhenium comes third (after osmium and iridium). It has a hexagonal crystal structure, whereas of the other strong metals only osmium and ruthenium do. Further, it has the rather * Rhenium, the last non-radioactive element to be discovered, is generally considered to be a very rare metal. It occurs principally as a by-product in the extraction of molybdenum, and annual production is a few tons. The present price is of the same order of magnitude as that of gold. Wear,
IO (1967)
313-318
314
E. RABIXOWICZ
high value of 1.615for its c/a ratio whereas osmium and ruthenium have c/a ratios” of 1.58. Thus, from a theoretical point of view rhenium clearly shows great promise as a material to be used in sliding applications. However, theory and practice may be widely different; accordingly a study has been carried out to evaluate rhenium’s friction and wear characteristics. APPARATUS
The equipment used in these tests was a pin-on-disk friction apparatus (Fig. I) which has been described previously7. The pin was a 0.6 cm diameter rod with a rounded end of diameter 0.6 cm, while the flat was a plate of dimensions 5cmx5cmxo.6cm.
Fig. 1. Schematic drawing of pin-on-disk friction apparatus
MATERIALS
Since the friction and wear properties of a material are rather variable, it is a standard practice not to measure only the absolute properties of the material under consideration, in this case rhenium, but also to carry on similar testing on other materials. In this case the metals used for comparison purposes in tests on friction and lubrication properties were chosen on the basis of their being hard common metals, namely iron (1020 steel), and nickel. Cobalt was also evaluated, since it resembles rhenium in having a hexagonal structure with high c/a ratio. Tests were also carried out on electroplated rhenium, and in this case the metals used for comparison were the hardest and, from the point of view of wear resistance, best metals capable of being electroplated, namely rhodium and chromium. All the electroplated metals were deposited on to the same substrate, namely nickel, the electroplated thickness being kept uniformly at 1.3.10-3 cm. Wear, 10 (1967) 313-318
FRICTION
AND
WEAR
PROPERTIES
OF
315
RHENIUM
In evaluating the wear resistance of unlubricated rhenium, tests were carried out on a large number of other metals. Whenever possible, the metals were selected as being of commercial purity grade, in the fully work-hardened condition. The Vicker’s microhardness values of all the metals were measured under a 50 g load, and the results are shown in Table I. TABLE
I
VICKERS
MICROHARDNESS
VALUES
OF METAL
Metal Flats Metal 1020
FLATS
ELECTRODEPOSITS
Electroplates Hardness (kglmma)
Metal
185
Nickel
208
Rhenium Chromium
Cobalt
335
Rhodium
Rhenium
507
Steel
AND
Hardness (kglmma) 494 525 1186
RESULTS
Friction values for clean and lubricated materials For these tests a 500 g load was used, the flat plate was rotated so as to give a sliding speed of 0.2 cm/set, and the friction was monitored with the dynamometer. Five states of lubrication were used, as follows: (I) Unlubricated. The two surfaces were abraded on 4/o grade emery paper in air, just before the tests, (2) Cetane. After cleaning the surfaces as above, a few drops of cetane, commercial purity grade, were introduced. (3) Pal&tic acid in cetane. The liquid at the interface consisted of a saturated solution of palmitic acid in cetane. (4) Fluorocarbon. The liquid at the interface consisted of a linear chlorotrifluorocarbon liquid, namely Halocarbon 13-21. (5) Silicone. The liquid at the interface consisted of a silicone oil, namely Dow Corning DC 200-100. The friction results are shown in Table II. According to these results, unlubricated rhenium, like unlubricated cobalt, TABLE
II
FRICTION
COEFFICIENTS
OF METAL
COMBINATIONS
Nickel
Cobalt
1020 Steel
Rhenium
Nickel
Cobalt
Rhenium
0%
on
on
0%
O?Z
0%
on
nickel
cobalt
1020 steel
rhenium
1020
Unlubricated
0.73
0.34
0.55
0.34
0.53
0.42
0.48
Cetane
0.43
0.16
0.36
0.15
0.18
0.41
0.14
Palmitic
steel
1020
steel
1020
steel
acid
in cetane
0.11
O.II
0.09
0.13
0.10
0.09
0.08
Fluorocarbon
0.11
0.1*
0.16
0.13
0.13
0.12
0.13
Silicone
0.32
0.24
0.41
0.20
0.25
0.21
0.53
Wear,
IO (1967)
313-318
gives below average friction because of its hexagonal structure; furthermore, lubricated rhenium appears quite normal in its ability to interact with boundary lubricants. None of the difficulties encountered with titanium7 is likely to arise when sliding systems involving lubricated rhenium surfaces are used. Wear values of udubricated
metals
For these tests the load used was 200 g, and the speed was IO cm/set. The times of the test varied from about 30 min to several days, until sufficient wear had taken place to be readily weighed on our balance, which was good to 10-4 g. In one case, the wear rate had to be estimated by the size of the scar diameter on the hemispherically ended rider. The results, expressed as gram worn per cm of sliding as shown in Table III.
TABLE WEAR
III
OF UNLUBRICATED
Rider
METALS
Flat
ll--ll_______ Nickel Cobalt Rhenium x020 Steel Nickel Rhenium
Nickel Cobalt Rhenium 102.0 Steel 1020 Steel 1020 Steel
Wear
of rider x
1010
Wear of flatX I010
Iglcm)
-. (dcm)
4500 2.8 0.7 4000 1000
3300
0.5
IO 1.0 2200 5500 670
It will be seen that the nickel and 1020 steel give high rates of wear, while the cobalt and rhenium give low rates of wear. Especially noteworthy is the fact that the rhenium wears less than the cobalt by a factor of 4 to IO on a weight basis. The factor is even greater on a volume basis, since the rhenium has more than twice the density of the cobalt.
Tests were carried out using hemis~herica~y ended riders of three steels, namely 1020 low carbon steel, 304 stainless steel, and 52100 bearing steel, and also copper, sliding against the three electroplated metals, chromium, rhodium and rhenium. For these tests the load was again 200 g and inspection of the surfaces suggests that the thin electroplated coatings were not damaged by this load. A speed of 20 cmjsec was used, and the duration of each test was one day, or several days, until a readily measurable loss of weight had occurred. The results of the tests are shown in Table IV. It is a little hard to evaluate these results. The rhenium gave the lowest friction of any of the plated metals, and the other metals slid against it suffered the least damage. The wear of the plates were pretty comparable especially if the wear is computed on a volume basis. It should be noted that the rhenium plate was mechanic~ly the least hard of all the plates tested, being even somewhat softer than the bulk rhenium. A harder rhenium plate might have performed even better. wear,
IO
(1967)
313-318
FRICTION AND WEAR
TABLE
PROPERTIES
317
OF RHENIUM
IV
FRICTION
AND
WEAR
OF ELECTROPLATED
MATERIALS
Weav of ridev X IOl” (g/cm)
Rider
Flat
Copper 1020 Steel 304 Steel 52100 Steel
Rhenium Rhenium Rhenium Rhenium
Copper 1020 Steel 304 Steel 52100 Steel
Chromium Chromium Chromium Chromium
Copper I020 Steel 304 Steel 52100 Steel
Rhodium Rhodium Rhodium Rhodium
plate plate plate plate
75 IO0 2.9 0.08
plate plate plate plate plate plate plate plate
200
500 23 8.3 350 40 1100
18
Wear of flat X IO10 (dcm)
Ftiction coefficient
I4 II I5 9.5
0.58 0.44 0.40 0.35
4.7 1.0 9.0 5.7
0.55 0.50 0.71 0.48
6.5 3.0 39
0.58 0.53 0.61 0.57
2.8
DISCUSSION All in all, the data are distinctly promising. Friction and wear data fully bear out the predictions made on the basis of the c/a ratio of rhenium. Furthermore, rhenium proves to be a metal which can be effectively lubricated by common liquid lubricants; even a silicone, which is ineffective on many metals, works quite well on rhenium.
WEAR
CONSTANT
=
2 x lO’3
IOC)-
IC)-
l HEXAGONAL
o OTHER
METALS METALS C:
I -
UNLUBRICATED METALS PIN ON DISK GEOMETRY LOAD 200 GRAMS
l Re
0.1
I 5
I IO
I
20 VICKERS
I I I 100 200 50 HARDNESS, kg/mm2
I 500
1000
Fig. 2. Volume wear rates as a function of hardness for like-metal sliding pairs. The straight line, near which most of the points lie, gives a value of ARCHARD~~wear coefficient of 2.10-9. Wear,
IO
(1967)
313-318
It may be of interest to relate the wear properties of rhenium to other metals. As part of a larger study, we have recently measured the wear rates of a large number of elemental metals using essentially the same conditions as those under which the data of Table III were generated. The results, expressed in units of 10-10 ~111s removed, per cm slid, under a 200 g load, are shown in Fig. 2. The low wear rates observed with rhenium, cobalt, and (surprisingly in view of its cubic structure) tungsten are quite striking. It is a little difficult to compare our data on the sliding behavior of rhenium with values obtained by others, since I have been unable to find any published data, though there may well have been some Russian work. GOODZEIT, HUNNICUTT ANI) ROACH~, who tested thirty-nine elemental metals against iron, did not use rhenium. BUCKLEY~, working in high vacua, finds rhenium very similar to cobalt, in agreement with our data. ACKNOWLEDGEMENTS
I am grateful to the Chase Brass and Copper Company of Solon, Ohio for supporting part of this work, and the American Chemical and Refining Company of Waterbury, Conn., for supplying electroplated test specimens. I am indebted to D. H. BUCKLEY for making his unpublished data available to me. REFERENCES I H. ERNST AND M. E. MERCHANT, Surface friction between metals-a basic factor in the metalcutting process, Proc. Special Summer Conf. Friction and Surface Finish, M.I.T. Press 1940, pp. 76-101. 2 M. SIKORSKI, Correlation of the coefficient of adhesion with various physical and mechanical properties of metal, Trans. ASME, 085 (1963) z7g-285. 3 E. P. KINGSBURY AND E. RABINOWICZ, Friction and wear of metals to IOOO’C, Trans. ASME, D~I (1959) 118-121. 4 D. H. BUCKLEY AND R. L. JOHNSON, Friction and wear of hexagonal metals and alloys as related to crystal structure and lattice parameters in vacuum, ASLE Trans., 9 (1966) 121-132. 5 E. RABINOWICZ, Friction alzd Wear of Materials, Wiley, New York, 1965, Chap. JI. 6 111.HANSEN, Constitution of Binary Alloys, McGraw-Hill, New York, 1958, Table B, Appendix. 7 E. RABINOWICZ, Frictional properties of titanium and its alloys, Metal Progr.. 65 (2) (1954) 107-110. 8 C. L. GOODZEIT, R. P. HUNNICUT AND A. E. ROACH, Frictional characteristics and surface damage of thirty-nine different elemental metals in sliding contact with iron, Trans. ASME.
78 (1956) 1669-1676. g D. H. BUCKLEY, personal communication. IO J. F. ARCHARD, Contact and rubbing of flat surfaces, J. .4@Z.
Wear,
IO (1967) 313-318
Phys.,
z# (1953) 981-988.
FRICTION
313
AND WEAR
PROPERTIES
OF RHENIUM
E. KABINOWICZ Department (U.S.A.)
of Mechanical
Engineering, Massachusetts
Institute
of Technology,
Cambridge,
Mass.
(Received November zg, 1966)
SUMMARY
Rhenium is a hard metal with a hexagonal crystal structure and high ratio of c to a lattice dimensions. Friction and wear tests show that, in agreement with theoretical predictions, rhenium gives low friction and very low wear, sliding either against itself or against other metals. Rhenium can be electroplated, and lubricants are effective on this metal. INTRODUCTION
It has been known since the classical work of ERNST AND MERCHANTI that metals with a hexagonal structure have unusual frictional properties, in that low values of friction coefficient values of about 0.5 are observed, even when such a metal slides on itself, unlubricated, in air. In contrast, other metals generally give friction coefficient values of about 1.0 under similar conditions. Later work by SIKORSKI~showed that hexagonal metals also give low values of adhesion coefficient, thus confirming the unusual and favorable position of hexagonal metals in sliding combinations. However, other studies suggested that the low friction-coefficient values for hexagonal metals like zinc and titanium were accompanied by quite high wear rates, thus throwing doubt on the value of these metals3. Recently, BUCKLEY AND JOHNSON~have shown that for low friction and low wear, a metal should have, not only a hexagonal crystal structure, but a high ratio of c to a dimensions in its crystal lattice. Thus cobalt, with a c/a ratio of 1.625, gives low friction, while titanium, with a lower c/a ratio of 1.58, has distinctly poorer friction properties. A little study of the structural properties of elemental metals shows up the promising position of the metal rhenium *. It is one of the strongest of all metals, as is shown by examining the values of the two indicative structure-insensitive properties of melting temperature and Young’s modulus 5. Rhenium’s melting temperature is second only to tungsten’s, while in terms of a high value of Young’s modulus rhenium comes third (after osmium and iridium). It has a hexagonal crystal structure, whereas of the other strong metals only osmium and ruthenium do. Further, it has the rather * Rhenium, the last non-radioactive element to be discovered, is generally considered to be a very rare metal. It occurs principally as a by-product in the extraction of molybdenum, and annual production is a few tons. The present price is of the same order of magnitude as that of gold. Wear,
IO (1967)
313-318
314
E. RABIXOWICZ
high value of 1.615for its c/a ratio whereas osmium and ruthenium have c/a ratios” of 1.58. Thus, from a theoretical point of view rhenium clearly shows great promise as a material to be used in sliding applications. However, theory and practice may be widely different; accordingly a study has been carried out to evaluate rhenium’s friction and wear characteristics. APPARATUS
The equipment used in these tests was a pin-on-disk friction apparatus (Fig. I) which has been described previously7. The pin was a 0.6 cm diameter rod with a rounded end of diameter 0.6 cm, while the flat was a plate of dimensions 5cmx5cmxo.6cm.
Fig. 1. Schematic drawing of pin-on-disk friction apparatus
MATERIALS
Since the friction and wear properties of a material are rather variable, it is a standard practice not to measure only the absolute properties of the material under consideration, in this case rhenium, but also to carry on similar testing on other materials. In this case the metals used for comparison purposes in tests on friction and lubrication properties were chosen on the basis of their being hard common metals, namely iron (1020 steel), and nickel. Cobalt was also evaluated, since it resembles rhenium in having a hexagonal structure with high c/a ratio. Tests were also carried out on electroplated rhenium, and in this case the metals used for comparison were the hardest and, from the point of view of wear resistance, best metals capable of being electroplated, namely rhodium and chromium. All the electroplated metals were deposited on to the same substrate, namely nickel, the electroplated thickness being kept uniformly at 1.3.10-3 cm. Wear, 10 (1967) 313-318
FRICTION
AND
WEAR
PROPERTIES
OF
315
RHENIUM
In evaluating the wear resistance of unlubricated rhenium, tests were carried out on a large number of other metals. Whenever possible, the metals were selected as being of commercial purity grade, in the fully work-hardened condition. The Vicker’s microhardness values of all the metals were measured under a 50 g load, and the results are shown in Table I. TABLE
I
VICKERS
MICROHARDNESS
VALUES
OF METAL
Metal Flats Metal 1020
FLATS
ELECTRODEPOSITS
Electroplates Hardness (kglmma)
Metal
185
Nickel
208
Rhenium Chromium
Cobalt
335
Rhodium
Rhenium
507
Steel
AND
Hardness (kglmma) 494 525 1186
RESULTS
Friction values for clean and lubricated materials For these tests a 500 g load was used, the flat plate was rotated so as to give a sliding speed of 0.2 cm/set, and the friction was monitored with the dynamometer. Five states of lubrication were used, as follows: (I) Unlubricated. The two surfaces were abraded on 4/o grade emery paper in air, just before the tests, (2) Cetane. After cleaning the surfaces as above, a few drops of cetane, commercial purity grade, were introduced. (3) Pal&tic acid in cetane. The liquid at the interface consisted of a saturated solution of palmitic acid in cetane. (4) Fluorocarbon. The liquid at the interface consisted of a linear chlorotrifluorocarbon liquid, namely Halocarbon 13-21. (5) Silicone. The liquid at the interface consisted of a silicone oil, namely Dow Corning DC 200-100. The friction results are shown in Table II. According to these results, unlubricated rhenium, like unlubricated cobalt, TABLE
II
FRICTION
COEFFICIENTS
OF METAL
COMBINATIONS
Nickel
Cobalt
1020 Steel
Rhenium
Nickel
Cobalt
Rhenium
0%
on
on
0%
O?Z
0%
on
nickel
cobalt
1020 steel
rhenium
1020
Unlubricated
0.73
0.34
0.55
0.34
0.53
0.42
0.48
Cetane
0.43
0.16
0.36
0.15
0.18
0.41
0.14
Palmitic
steel
1020
steel
1020
steel
acid
in cetane
0.11
O.II
0.09
0.13
0.10
0.09
0.08
Fluorocarbon
0.11
0.1*
0.16
0.13
0.13
0.12
0.13
Silicone
0.32
0.24
0.41
0.20
0.25
0.21
0.53
Wear,
IO (1967)
313-318
gives below average friction because of its hexagonal structure; furthermore, lubricated rhenium appears quite normal in its ability to interact with boundary lubricants. None of the difficulties encountered with titanium7 is likely to arise when sliding systems involving lubricated rhenium surfaces are used. Wear values of udubricated
metals
For these tests the load used was 200 g, and the speed was IO cm/set. The times of the test varied from about 30 min to several days, until sufficient wear had taken place to be readily weighed on our balance, which was good to 10-4 g. In one case, the wear rate had to be estimated by the size of the scar diameter on the hemispherically ended rider. The results, expressed as gram worn per cm of sliding as shown in Table III.
TABLE WEAR
III
OF UNLUBRICATED
Rider
METALS
Flat
ll--ll_______ Nickel Cobalt Rhenium x020 Steel Nickel Rhenium
Nickel Cobalt Rhenium 102.0 Steel 1020 Steel 1020 Steel
Wear
of rider x
1010
Wear of flatX I010
Iglcm)
-. (dcm)
4500 2.8 0.7 4000 1000
3300
0.5
IO 1.0 2200 5500 670
It will be seen that the nickel and 1020 steel give high rates of wear, while the cobalt and rhenium give low rates of wear. Especially noteworthy is the fact that the rhenium wears less than the cobalt by a factor of 4 to IO on a weight basis. The factor is even greater on a volume basis, since the rhenium has more than twice the density of the cobalt.
Tests were carried out using hemis~herica~y ended riders of three steels, namely 1020 low carbon steel, 304 stainless steel, and 52100 bearing steel, and also copper, sliding against the three electroplated metals, chromium, rhodium and rhenium. For these tests the load was again 200 g and inspection of the surfaces suggests that the thin electroplated coatings were not damaged by this load. A speed of 20 cmjsec was used, and the duration of each test was one day, or several days, until a readily measurable loss of weight had occurred. The results of the tests are shown in Table IV. It is a little hard to evaluate these results. The rhenium gave the lowest friction of any of the plated metals, and the other metals slid against it suffered the least damage. The wear of the plates were pretty comparable especially if the wear is computed on a volume basis. It should be noted that the rhenium plate was mechanic~ly the least hard of all the plates tested, being even somewhat softer than the bulk rhenium. A harder rhenium plate might have performed even better. wear,
IO
(1967)
313-318
FRICTION AND WEAR
TABLE
PROPERTIES
317
OF RHENIUM
IV
FRICTION
AND
WEAR
OF ELECTROPLATED
MATERIALS
Weav of ridev X IOl” (g/cm)
Rider
Flat
Copper 1020 Steel 304 Steel 52100 Steel
Rhenium Rhenium Rhenium Rhenium
Copper 1020 Steel 304 Steel 52100 Steel
Chromium Chromium Chromium Chromium
Copper I020 Steel 304 Steel 52100 Steel
Rhodium Rhodium Rhodium Rhodium
plate plate plate plate
75 IO0 2.9 0.08
plate plate plate plate plate plate plate plate
200
500 23 8.3 350 40 1100
18
Wear of flat X IO10 (dcm)
Ftiction coefficient
I4 II I5 9.5
0.58 0.44 0.40 0.35
4.7 1.0 9.0 5.7
0.55 0.50 0.71 0.48
6.5 3.0 39
0.58 0.53 0.61 0.57
2.8
DISCUSSION All in all, the data are distinctly promising. Friction and wear data fully bear out the predictions made on the basis of the c/a ratio of rhenium. Furthermore, rhenium proves to be a metal which can be effectively lubricated by common liquid lubricants; even a silicone, which is ineffective on many metals, works quite well on rhenium.
WEAR
CONSTANT
=
2 x lO’3
IOC)-
IC)-
l HEXAGONAL
o OTHER
METALS METALS C:
I -
UNLUBRICATED METALS PIN ON DISK GEOMETRY LOAD 200 GRAMS
l Re
0.1
I 5
I IO
I
20 VICKERS
I I I 100 200 50 HARDNESS, kg/mm2
I 500
1000
Fig. 2. Volume wear rates as a function of hardness for like-metal sliding pairs. The straight line, near which most of the points lie, gives a value of ARCHARD~~wear coefficient of 2.10-9. Wear,
IO
(1967)
313-318
It may be of interest to relate the wear properties of rhenium to other metals. As part of a larger study, we have recently measured the wear rates of a large number of elemental metals using essentially the same conditions as those under which the data of Table III were generated. The results, expressed in units of 10-10 ~111s removed, per cm slid, under a 200 g load, are shown in Fig. 2. The low wear rates observed with rhenium, cobalt, and (surprisingly in view of its cubic structure) tungsten are quite striking. It is a little difficult to compare our data on the sliding behavior of rhenium with values obtained by others, since I have been unable to find any published data, though there may well have been some Russian work. GOODZEIT, HUNNICUTT ANI) ROACH~, who tested thirty-nine elemental metals against iron, did not use rhenium. BUCKLEY~, working in high vacua, finds rhenium very similar to cobalt, in agreement with our data. ACKNOWLEDGEMENTS
I am grateful to the Chase Brass and Copper Company of Solon, Ohio for supporting part of this work, and the American Chemical and Refining Company of Waterbury, Conn., for supplying electroplated test specimens. I am indebted to D. H. BUCKLEY for making his unpublished data available to me. REFERENCES I H. ERNST AND M. E. MERCHANT, Surface friction between metals-a basic factor in the metalcutting process, Proc. Special Summer Conf. Friction and Surface Finish, M.I.T. Press 1940, pp. 76-101. 2 M. SIKORSKI, Correlation of the coefficient of adhesion with various physical and mechanical properties of metal, Trans. ASME, 085 (1963) z7g-285. 3 E. P. KINGSBURY AND E. RABINOWICZ, Friction and wear of metals to IOOO’C, Trans. ASME, D~I (1959) 118-121. 4 D. H. BUCKLEY AND R. L. JOHNSON, Friction and wear of hexagonal metals and alloys as related to crystal structure and lattice parameters in vacuum, ASLE Trans., 9 (1966) 121-132. 5 E. RABINOWICZ, Friction alzd Wear of Materials, Wiley, New York, 1965, Chap. JI. 6 111.HANSEN, Constitution of Binary Alloys, McGraw-Hill, New York, 1958, Table B, Appendix. 7 E. RABINOWICZ, Frictional properties of titanium and its alloys, Metal Progr.. 65 (2) (1954) 107-110. 8 C. L. GOODZEIT, R. P. HUNNICUT AND A. E. ROACH, Frictional characteristics and surface damage of thirty-nine different elemental metals in sliding contact with iron, Trans. ASME.
78 (1956) 1669-1676. g D. H. BUCKLEY, personal communication. IO J. F. ARCHARD, Contact and rubbing of flat surfaces, J. .4@Z.
Wear,
IO (1967) 313-318
Phys.,
z# (1953) 981-988.